The present invention relates to an image forming apparatus configured to carry out an electrophotographic image forming process, such as a copier, printer or facsimile apparatus, a printing method applicable to such an image forming apparatus, and a computer program for implementing a printing process carried out by such an image forming apparatus.
An electrophotographic image forming apparatus is configured to form an image by a process including: forming an electrostatic latent image by exposing a surface of a photosensitive member in an electrostatically charged condition to signal light in accordance with image information; forming a toner image by supplying toner to the electrostatic latent image; and transferring and fixing the toner image to a recording medium.
In fixing the toner image onto the recording medium, a fixing device is used which includes, for example, a fixing roller and a pressurizing roller pressed against the fixing roller. The recording medium introduced between pressure-contact portions of the fixing roller and pressurizing roller is heated and pressurized, so that the toner image is fixed onto the recording medium.
To meet growing printing speed, a method of increasing the amount of heat or a method of increasing the pressure to increase the fixing nip pressure is applied to the fixing device. Increasing the amount of heat, however, is against the recent energy saving trend and, in addition, raises a problem that a printing sheet just delivered is so hot that a human cannot touch the printing sheet. As a result, cooling means, such as a cooling fan, becomes necessary.
In recent color printing, toners for respective colors (i.e., yellow, magenta, cyan and black) are stacked on a recording medium. The thickness of a toner stack of three color toners developing black is about 40 μm (inclusive of air), whereas the thickness of one monochrome toner layer is about 14 μm. In order to fix such a toner stack having particle diameters of about 6 μm onto the recording medium by the heat of the fixing roller surface, it is necessary to increase the heat and the pressure, as well as to remove air having high heat insulation to reduce the spacing between each toner particle and the fixing roller. This results in likelihood of the occurrence of toner offset in color printing in particular. Under actual circumstances, the occurrence of toner offset is suppressed by remedies including: adding wax to the toners or increasing the amount of wax to be added; and using a fluoro-type tube for the fixing roller. Since a toner in a state before fixing bears air, color printing requires a larger amount of heat and a higher pressure than monochrome printing.
Printing sheets as recording media include those containing rough fibers and those having smooth surfaces such as a coated paper sheet. In fixing a toner to a printing sheet containing rough fibers, the printing sheet calls for a larger amount of heat and a higher pressure as its texture becomes rougher.
Therefore, the image forming apparatus is configured to change the fixing conditions in accordance with the type of recording media to be used. Specifically, the fixing temperature in a thin sheet mode is a preset fixing temperature lower than a normal fixing temperature because the amount of heat to be applied can be reduced, while the printing speed in a thick sheet mode is preset lower than a normal printing speed because the amount of heat for fixing is insufficient.
In ensuring a usual fixing performance on plain paper sheets in general, the fixing temperature has to be set within a range from 180° C. to 190° C. However, the temperature of the fixing roller has to be raised to such a fixing temperature from about room temperature in a state assumed just after power has been turned ON or in a state of being resumed from a sleep mode and, hence, a considerable warm-up time is necessary in such a case. In order to shorten such a warm-up time, measures have been taken which include: changing the fixing system from a roller type system to a belt type system; and lowering the melting point of a toner. Alternative measures to shorten the warm-up time having been taken include adding an auxiliary lamp to be used at the time the fixing temperature starts rising. Such measures are forced to increase the power consumption.
An electrophotographic apparatus described in Japanese Patent Laid-Open Publication No. 2001-356616 is configured to shorten the fixing roller warm-up time by shortening the start-up waiting time before printing on a small number of printing sheets ranging from one to a few sheets in order to make a first printing time earlier.
Users, in general, wish the printing operation to start promptly in the state assumed just after power has been turned ON or in the state of being resumed from the sleep mode. If the apparatus assumes a state in which image forming processing is inhibited (i.e., standby state) in spite of a user wishing prompt copying or data output, the user will be displeased.
If a state in which an instruction to proceed with the image forming process can be accepted (i.e., ready state) is assumed before the preset fixing temperature is reached, the user will not be displeased. Thus, an image forming apparatus satisfying the user can be provided.
Actual printing jobs mostly include jobs of performing printing on a few printing sheets at most, though there are a few jobs of performing printing on a large number of printing sheets up to several hundred printing sheets. For this reason, the user is fed up with such a long waiting time very much.
The electrophotographic apparatus described in Japanese Patent Laid-Open Publication No. 2001-356616, which is capable of early printing for the purpose described above, is based on the precondition that the number of printing sheets to be subjected to printing is small and hence is incapable of accommodating a large number of printing sheets to be used for printing.
In view of the foregoing problems, a feature of the present invention is to provide an image forming apparatus which is capable of starting image formation early while enabling an image forming process to be carried out on a large number of printing sheets.